Synthesis of CdS and CdSe nanocrystallites using a novel single-molecule precursors approach

The synthesis of CdS and CdSe nanocrystallites using the thermolysis of several dithioor diselenocarbamato complexes of cadmium in trioctylphosphine oxide (TOPO) is reported. The nanodispersed materials obtained show quantum size effects in their optical spectra and exhibit near band-edge luminescence. The influence of experimental parameters on the properties of the nanocrystallites is discussed. HRTEM images of these materials show well-defined, crystalline nanosized particles. Standard size fractionation procedures can be performed in order to narrow the size dispersion of the samples. The TOPO-capped CdS and CdSe nanocrystallites and simple organic bridging ligands, such as 2,2¢-bipyrimidine, are used as the starting materials for the preparation of novel nanocomposites. The optical properties shown by these new nanocomposites are compared with those of the starting nanodispersed materials

Spin dynamics in semiconductor nanocrystals

Time-resolved Faraday rotation is used to study both transverse and longitudinal spin relaxation in chemically-synthesized CdSe nanocrystals (NCs) 22-80 Angstroms in diameter. The precession of optically-injected spins in a transverse magnetic field occurs at distinct frequencies whose assignment to electron and exciton spins is developed through systematic studies of the size-dependence and theoretical calculations. It is shown that the transverse spin lifetime is limited by inhomogeneous dephasing to a degree that cannot be accounted for by the NC size distribution alone. Longitudinal spin relaxation in these NCs occurs on several distinct timescales ranging from 100 ps-10 microseconds and exhibits markedly different dependencies on temperature and field in comparison to transverse spin relaxation.Comment: 25 pages, 11 figures, tabl

Epigenetics and developmental programming of welfare and production traits in farm animals

The concept that postnatal health and development can be influenced by events that occur in utero originated from epidemiological studies in humans supported by numerous mechanistic (including epigenetic) studies in a variety of model species. Referred to as the ‘developmental origins of health and disease’ or ‘DOHaD’ hypothesis, the primary focus of large-animal studies until quite recently had been biomedical. Attention has since turned towards traits of commercial importance in farm animals. Herein we review the evidence that prenatal risk factors, including suboptimal parental nutrition, gestational stress, exposure to environmental chemicals and advanced breeding technologies, can determine traits such as postnatal growth, feed efficiency, milk yield, carcass composition, animal welfare and reproductive potential. We consider the role of epigenetic and cytoplasmic mechanisms of inheritance, and discuss implications for livestock production and future research endeavours. We conclude that although the concept is proven for several traits, issues relating to effect size, and hence commercial importance, remain. Studies have also invariably been conducted under controlled experimental conditions, frequently assessing single risk factors, thereby limiting their translational value for livestock production. We propose concerted international research efforts that consider multiple, concurrent stressors to better represent effects of contemporary animal production systems

Density functional study on the morphology and photoabsorption of CdSe nanoclusters

The geometrical and electronic structures of a series of small CdSe quantum dots protected by various ligands have been studied by density functional theory. UV-vis spectra have been calculated by time-dependent density functional theory (TDDFT). The goal of this investigation is the rationalization of the basic properties of these systems, in particular, the nature of the exciton peaks. This study has been focused on the (CdSe)(x), x = 13, 19, 33, and 66, "magic-size" clusters that are characterized by high stability and large optical gaps. The geometries of the cluster are relaxed both in vacuum and in the presence of the surfactant ligands. To describe the interaction between the bare clusters and the surfactants, model types of ligands are introduced: fatty acids are modeled using formic and acetic acid and amines are modeled using ammonia and methyl amine. Present calculations demonstrate that the ligands play a crucial role in stabilizing the structure in a bulklike geometry and strongly affect the optical gap of the clusters, due to an optimal coordination of the surface atoms. For these "magic-size" clusters, the UV-vis spectrum is calculated at the TDDFT level. The calculated spectra are in good agreement with the experimental ones for clusters with the same dimension capped with the same type of Uganda. This suggests that our structures are realistic models of the actual quantum dots